1. Field of the Invention
The present invention relates to a thin film electroluminescent (EL) device which emits in response to the application of an electric field, and more particularly to a thin film EL device which has brightness vs. applied voltage characteristics involving hysteresis, i.e., which has a memory function.
2. Description of the Prior Art
Ever since a thin film EL device was developed for giving a bright EL by applying an a.c. electric field to zinc sulfide (ZnS) doped with luminescent center, various investigations have been made on the structure of such devices. Thin film EL devices comprising an emitting layer of ZnS doped with Mn for luminescent centers, insulating layers sandwiching the emitting layer, and electrodes sandwiching the assembly and including at least one transparent electrode have been made commercially available as light weight, compact and thin EL display panels because of their high brightness and long life characteristics.
Thin film EL devices are also developed which incorporate a controlled amount of the dopant Mn and which are thereby adapted to exhibit a hysteresis phenomenon (memory effect) such that a different brightness can be obtained at the same voltage value when the applied voltage increases and decreases. Since these devices are also adapted for writing and erasing with light and heat, research on practical application is promoted on the use of the display devices as multipurpose input-output terminal systems.
However, the memory effect is available usually only when the thin film EL device uses a ZnS emitting layer doped with Mn luminescent centers, such that the color of luminescence is limited only to yellowish-orange specific to Mn. The device is therefore not fully useful as a display terminal. To explore applications to wider use, continuous research is conducted on thin film EL devices having a memory effect in other luminescence colors. For example, research efforts have been directed to the use of CaS, SrS and like sulfides of alkaline earth metals as host materials for the emitting layer (see S. Tanaka et al., "Multi-Color Electroluminescence in Alkaline-Earth-Sulfide Thin-Film Devices," SID 85 DIGEST, p218, May 1985).
Nevertheless, satisfactory devices still remain to be realized.